DESCRIPTION: The overall goal of this applicaton is to obtain a better understanding of the HIV pathogenesis which leads to rapid disease progression in infected children. A subset of HIV-infected children have much more rapid disease progression than adults for still unexplained reasons. The applicants postulate that some young children with early and rapid disease progression may have early disruption of the thymic microenvironment by HIV, resulting in a reduced reservoir of post-thymic lymphocytes. The heavy demand of CD4+ cell regeneration to compensate for HIV-induced destruction leads to rapid exhaustion of this reduced reservoir and early disease progression. Supporting this hypothesis are the applicants' observations that: (1) there is a marked decrease in the CD4+RA+("naive") T lymphocytes in some infected infants, suggesting an intrathymic viral effect; and (2) that many of the earliest progressors of pediatric AIDS have pre-AIDS immunophenotypic profiles similar to those found in children with the DiGeorge congenital thymic anomaly (decrease in CD4+ T cells, as well as in CD8+ T cells and CD5+ B cells). Work in SCID-hu mouse model done by other researchers suggests that there are divergent effects on human thymocyte maturation by HIV-1, depending on the viral strain. The applicants postulate that HIV-1 strains from this subset of rapid progressors may be more disruptive to the thymic microenvironment. There are four specific aims described: (1) to study prospectively immunophenotypic profiles to identify cohort of infants with patterns consistent with early thymic defects (CD4+, CD8+, CD5+ decline) and compare the disease course of these infants with others with more common profiles (CD4+ decline, CD8+ increase); (2) to compare infection of thymic tissue and disruption of the thymic environment observed in vitro with viral isolates from infants and children with early versus slow progression; (3) to compare the effect of infection with different isolates on the maturation of CD34+ stem cells in thymic culture; and (4) to correlate the observations regarding immunophenotypic profiles and thymic culture with measures of viral burden, neutralizing antibody, and induction of CD69 expression on CD8+ cells following HIV antigen exposure as a marker for HIV-specific CD8+ lymphocyte responses. In the proposed studies, the investigators will attempt to identify rapid progressors, particularly children with immunophenotypic profile consistent with early thymic defects and will evaluate potential differences of their HIV-1 strains from that of delayed progressors. Primary viral isolates will be compared for effects on fetal, neonatal and young infant thymic tissues, using an in vitro model of thymic slices cultures, with or without stem cell co-cultures. Difference in disruption of the thymic microenvironment, and ability to support thymic maturation of lymphocytes from stem cells will be assessed by immunohistochemical and in situ molecular methods. Viral strains with distinct patterns of thymic "virulence" will be analyzed for genetic markers using nucleotide sequence data that have been obtained on these viral isolates. Correlation studies will include quantitation of viral load, neutralizing antibodies and HIV-specific CD8+ lymphocyte responses (using novel FACS analyses of CD69 activation markers). Identifying the mechanism for rapid disease in children is important for designing therapeutic strategies, including possibly thymic transplantation.